Modern commercial refrigeration systems include means for detection of malfunction which could be detrimental to stored commodities. Thus it has long been generally recognized that there is an important ever-present risk of spoilage of foods, pharmaceuticals, biological products or other chemical materials due to unattended elevated temperatures of extended duration. Prior art instruments intended to meet that need have taken a variety of forms. One type, which is not reusable, detects warm up above a preselected temperature and records the duration of the warm up period. Another type is reusable and records the maximum temperature, but does not indicate the length of time at temperature. Moreover, none of the prior art devices is capable of indicating either variations in the rate of warm up or the length of time at intermediate temperature in the warm up temperature range. From the standpoint of waste prevention and needless discard of refrigerated materials such data and information would obviously be very desirable.